Variable angle reflectance is an important spectroscopic technique. Certain samples, such as opaque substances, films on opaque substrates, and films on liquids, are tedious or practically impossible to analyze with conventional transmission spectroscopy equipment. The analyses of such samples by reflection spectroscopy, however, are straightforward.
There are three different methods of reflection spectroscopy. The technique employed depends on the nature of the sample and the information sought. External reflection spectroscopy is applicable to thin films on opaque substrates and opaque smooth solids. Internal reflectance permits spectral measurements of liquids, powders, pastes, gels, and soft solids. Diffuse reflectance is most commonly used for the analysis of powders and rough surface solids. Variable angle studies, using either external or internal reflection spectroscopy, provide data for the determination of optical constants and sample thicknesses.
The spectrometer accessories currently available generally perform only one of the three reflection techniques. Furthermore, most variable angle accessories are only useable over a limited range of angles. They also do not maintain alignment and do not reflect the incident radiation from the same sample area when the incident angle is changed. For further backgound on the state of the art of this technology, reference is had to U.S. Pat. No. 3,603,690, and the Harrick Scientific Corporation catalog HSC-831 which describes a number of the currently available reflection accessories.